Thomas Andrew Tresser is a vigilante, turned operative for the U.S. government and a master of disguise. His default costume is a black turtleneck sweater with a balance as a chest symbol and a chest holster.
Thomas Andrew Tresser was an applicant to an unnamed government agency whose brother Craig was an undercover agent infiltrating a criminal syndicate called "the Council". Craig was brainwashed into killing their family friend Ben Williams and was subsequently killed in self-defense by fellow agents.
Tom thus became "Nemesis", preferring to use an alias instead of his dishonored family name. With the assistance of Batman, he cleared his brother's name and saw the men responsible for his brother's death dead. [source: wiki]
Nemesis (Thomas Andrew Tresser) appeared as a background character in several episodes of Justice League Unlimited.
He joined the Justice League after the Thanagarian invasion. He debuted in the first episode of Justice League Unlimited ("Initiation"), and later was seen more prominently in the episode "Dark Heart".